Substrate soil preparation
In the present study, the substrate soil for P. massoniana growth was collected from southern forest area of China where P. massoniana are naturally distributed. The soil was subjected to a Ca leaching process to minimize the soil Ca content according to previous report.21 Then soil nutrients were recovered through a two months nutrient solution supplementation with modified Hoagland solution containing different Ca concentrations (20.0, 2.0, or 0.1 mM), respectively.17 The restored soil samples were used for the following P. massonianaseedling cultivation at high Ca level (20.0 mM), medium Ca level (2.0 mM) and low Ca level (0.1 mM), respectively. The final soil pH was approximate 5.0 for all the prepared substrate soil.